The present invention relates to a control system for an internal combustion engine having a variable control mechanism for variably controlling engine performance characteristics according to an engine operating condition. More specifically, the present invention relates to a control system and method for an internal combustion engine having a variable valve operating mechanism capable of varying at least one of a valve lift and an operating angle of an engine valve (i.e., an intake valve or an exhaust valve) in accordance with an engine operating condition.
Heretofore, the assignee of this application proposed a control system for an internal combustion engine having a variable valve operating mechanism, which will be described briefly in the following.
The variable valve operating mechanism is adapted for use with an intake valve and includes an eccentric cam fixedly attached to a drive shaft that is rotatable in timed relation with a crank shaft, and an oscillation cam having a cam surface in sliding contact with an upper surface of a valve lifter provided to an upper end portion of the intake valve and driven by the drive cam by way of a multi-link type transmission mechanism to open and close the intake valve.
The above-described transmission mechanism includes a rocker arm disposed above the oscillation cam and mounted on a control shaft for oscillation motion, a pivotal link having an annular base portion rotatably mounted on a circular external surface of the eccentric cam and a protruded arm portion pivotally connected to an end portion of the rocker arm, and a connecting rod having an end portion pivotally connected to another end portion of the rocker arm and another end portion pivotally connected to a cam nose side end portion of the oscillation cam.
Further, fixedly mounted on the control shaft is an eccentric control cam having a geometric or oscillation center that is offset from a rotational axis of the control shaft. The rocker arm is mounted at a central portion thereof on the control cam. The control shaft is thus capable of varying the oscillation center of the rocker arm depending upon a variation of rotational position or phase thereof, thereby varying the position at which the oscillation cam is in sliding contact with the upper surface of the valve lifter and variably controlling the valve lift and operation angle of the intake valve.
Namely, when the engine operating condition is, for example, in a low speed-low load range, the control shaft is rotated in one direction by an actuator that is constituted by an electric motor, thus causing the control shaft to rotate in the same direction and thereby moving the pivot center of the rocker arm to move in one direction. By this, the pivot center of the joint between the rocker arm and the link arm and the pivot center of the joint between the rocker arm and the connecting rod are displaced so as to pull the cam nose side end portion of the oscillation cam upward and thereby cause the position at which the oscillation cam is brought into contact with the upper surface of the valve lifter to move toward the base circle side. Accordingly, the valve lift characteristics of the intake valve is controlled so as to attain a small lift.
On the other hand, when the engine operating condition changes into a high speed-high load range, the electric motor is driven in the opposite direction, thus allowing the control cam to be driven in the opposite direction and into a predetermined position by the control shaft and thereby causing the oscillation center of the rocker arm to move in the opposite direction. By this, the cam nose side end portion of the oscillation cam is pushed down by means of the link arm, thus causing the position at which the oscillation cam is in contact with the upper surface of the valve lifter to move toward the cam nose side and thereby controlling the valve lift characteristics of the intake valve so as to attain a large lift.
Accordingly, the control system can improve the engine performance and efficiency, such as fuel consumption and output, in accordance with the engine operating condition.